(a) Field of the Invention
The present invention relates to a method of forming a trench for semiconductor device isolation and, more particularly, to a method of forming a trench for semiconductor device isolation which exhibits good performance characteristics well adapted for serving in device miniaturization.
(b) Description of the Related Art
Generally, a local oxidation of silicon (LOCOS) technique has been used for device isolation purpose in the semiconductor fabrication process.
In the LOCOS technique, the silicon wafer itself is thermally oxidized while using a nitride for a mask, and requires only a small number of processing steps. For this reason, the resulting oxide involves lower degree of stress, and exhibits good insulating characteristics.
However, in the application of the LOCOS technique, the device isolation area takes a large volume that limits device miniaturization as well as involves occurrence of the so-called bird's beak.
In order to solve the above problems, a shallow trench isolation (STI) technique has been suggested.
In the STI technique, a shallow trench is made in the silicon wafer, and filled with an insulating material. In this structure, the device isolation area is small, and hence the resulting trench can be well adapted for device miniaturization.
FIGS. 1A to 1E schematically illustrate the steps of processing a shallow trench for device isolation according to a prior art.
As shown in FIG. 1A, a silicon wafer 1 is sequentially overlaid with a layer of pad oxide 2 and a layer of nitride 3. The pad oxide 2 is to relieve stress occurring between the silicon wafer 1 and the nitride 3. A photoresist film is coated onto the silicon wafer 1 over the pad oxide 2 and the nitride 3. The photoresist film is exposed to light through a patterning mask, and developed to thereby form a photoresist pattern 4.
As shown in FIG. 1B, the patterned silicon wafer 1 is put in a dry etching chamber, and the nitride 3 and the pad oxide 2 exposed through the mask pattern 4 are etched. Furthermore, the exposed silicon wafer 1 is also etched by a predetermined depth to thereby form a shallow trench at the device isolation area.
As shown in FIG. 1C, the photoresist pattern 4 is removed from the nitride 3, and the silicon wafer 1 is cleaned. Thereafter, the silicon wafer 1 is put in a furnace, and undergoes thermal oxidation therein at high temperature for a long time. As a result, a layer of densified liner oxide 5 having a thickness of several hundred angstroms or more is formed at the inner wall of the trench.
As shown in FIG. 1D, a large thickness of insulating layer 6 is deposited onto the entire surface of the silicon wafer 1 through chemical vapor deposition (CVD) such that the inside of the trench is completely buried by the insulating layer 6.
As shown in FIG. 1E, the insulating layer 6 is annealed and densified, and a photoresist film is then coated on the densified insulating layer 6. The photoresist film is exposed to light through a reverse mask that has a pattern reverse to that of the aforementioned patterning mask, and developed to form a photoresist pattern. The insulating layer 6 is etched by using the photoresist pattern such that the insulating layer 6 is left only at the trench area. The photoresist pattern is then removed, and the insulating layer 6 is planarized through chemical mechanical polishing (CMP) while utilizing the nitride 3 as a stopping layer for the polishing. In this way, the formation of a shallow trench for device isolation is completed.
In the trench formed by using the above-described technique, electric field is concentrated on the side wall edge corner of the trench at the device driving, and the edge corner exhibits weakness in channel leakage such as a stress-induced leakage (SILC). In this respect, it becomes necessary that the side wall edge corner of the trench should be rounded to endure against leakage.
In order to achieve such a corner rounding, chlorine (Cl) is conventionally used for the liner oxide formation step such that the side wall edge corner of the trench is etched during the thermal oxidation.
However, in the above technique, the corner rounding is insufficiently made, and chlorine remains at the interface between the silicon wafer and the liner oxide so that the contamination due to the chlorine content induces leakage in the presence of subsequent thermal processing steps.